`MailboxesResource.create()` missing required `agent_handle` parameter

[alex-w-99]

The POST /mailboxes endpoint requires agent_handle in the request body, but MailboxesResource.create() does not accept or pass it. This causes a 422 when creating a mailbox through the SDK.

inkbox.mail.exceptions.InkboxAPIError: HTTP 422: [{'type': 'missing', 'loc': ['body', 'agent_handle'], 'msg': 'Field required', 'input': {'display_name': 'inkbox-kernel Agent'}}]

AgentIdentity.create_mailbox() is also affected since it delegates to MailboxesResource.create() without passing self.agent_handle.

Suggested fix: add agent_handle as a required param to MailboxesResource.create(), and have AgentIdentity.create_mailbox() pass self.agent_handle automatically.